This application claims the priority from Korean Patent Application No. 2003-21919, filed on Apr. 8, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to metallic nickel powders and a method of preparing the same. The present invention also relates to a multi-layer ceramic capacitor (MLCC).
2. Description of the Related Art
Multi-layer ceramic capacitors (MLCCs) are stacked structures of multiple thin dielectric layers and multiple inner electrodes. Such MLCCs can achieve a large capacitance even in a small volume, and thus, have been widely used in various electronic equipments such as computers and mobile communication devices.
An Ag—Pd alloy has been used as a material for inner electrodes of MLCCs. Due to its sinterability in the air, the Ag—Pd alloy can be easily utilized in manufacturing MLCCs. However, the Ag—Pd alloy is costly. Since the late 1990s, to decrease the manufacturing cost of MLCCs, inexpensive nickel has been used as a material for inner electrodes. A nickel inner electrode for MLCCs is made from a conductive paste containing metallic nickel powders.
Metallic nickel powders are prepared by a gas phase method or a liquid phase method. Due to relatively easy control of the shapes of metallic nickel powders and impurities, the gas phase method has been widely used. However, the gas phase method is unfavorable for fine powder formation and mass production. On the other hand, the liquid phase method is favorable for mass production, and its initial capital and process costs are low.
The liquid phase method is classified into two types. In the first type, nickel hydroxide is used as a starting material to be converted into metallic nickel powders. In the second type, a nickel precursor material, such as nickel salt and nickel oxide, except nickel hydroxide, is used as a starting material to be converted into metallic nickel powders.
The first type is relatively simple. However, there are disadvantages in that nickel hydroxide, which is the staring material, is expensive and the particle size of metallic nickel powders is not easily controlled.
Even though the second type is relatively complicated, there are advantages in that an inexpensive nickel precursor material such as nickel sulfate, nickel chloride, and nickel acetate can be used as the starting material and the particle size of powders can be easily controlled in the range of several nanometers to several hundreds nanometers.
Illustrative examples of the liquid phase method are disclosed in U.S. Pat. Nos. 4,539,041 and 6,120,576.
U.S. Pat. No. 4,539,041 discloses a method for preparing metal powders by dispersing the oxide, hydroxide, or salt of gold, palladium, platinum, iridium, osmium, copper, silver, nickel, cobalt, lead, or cadmium in liquid polyol which is a reducing agent, followed by heating.
U.S. Pat. No. 6,120,576 discloses a method for preparing metallic nickel powders, which includes the steps of mixing an aqueous sodium hydroxide solution with an aqueous solution of nickel sulfate to produce nickel hydroxide; reducing the resultant nickel hydroxide with hydrazine to produce nickel; and recovering the nickel thus produced.
Nickel hydroxide is often used as a starting material or an intermediate in the liquid phase method because it is easily reduced, relative to other types of nickel compounds.
In the second type, an alkaline is added to convert a nickel precursor compound to nickel hydroxide. The alkaline that is generally used is either sodium hydroxide (NaOH) or potassium hydroxide (KOH). In this case, impurities such as sodium (Na) and potassium (K) may remain on the surfaces of metallic nickel powders. Since sodium and potassium have low surface energy, they are not easily removed from the metallic nickel powders.
Metallic nickel powders for high capacitance MLCCs must have more enhanced electric conductivity and contain a smaller quantity of impurities that adversely affect the capacitance of dielectrics. An alkaline metal such as sodium and potassium lowers both the electric conductivity of nickel electrodes and the capacitance of dielectrics.